Abrasive flow polishing is a process that may be used for the surface polishing of metal parts prior to their distribution. The method has been found to be advantageous for the surface finishing and polishing of manufactured parts having complex structural features such as internal passages and/or buried cavities that are difficult to access by other surface finishing techniques. The process may take place in an abrasive flow machine that passes a thick abrasive media back and forth over the surfaces and through any internal passages and/or cavities of a part. The abrasive flow machine may have a fixture to hold the part in a cylinder and it may have pistons to pump the abrasive media back and forth over the part being retained in position by the fixture.
Abrasive flow polishing has been employed as a manufacturing step in the production of surface finished airfoil clusters for gas turbine engines. The airfoil clusters may consist of a plurality of airfoils attached to a supporting rail to form a unitary structure. Due to the complex structural features of airfoil clusters, surface polishing by abrasive flow machining may be more effective than some other polishing methods which may fail to polish hard to reach surfaces of the airfoil cluster to a desired degree.
While abrasive flow machining of airfoil clusters may be an effective method for the surface polishing of airfoil clusters, differential finishing (or uneven surface polishing) of airfoil clusters may occur in some cases. In particular, current fixtures for abrasive flow polishing of airfoil clusters are designed to hold the part in position but may do little to control and guide the flow and velocity of the abrasive media over hard to reach areas of the part. As a result, certain surfaces of the airfoil cluster may receive more surface polishing and more difficult to reach surfaces may be left with non-conforming surface roughness. The hard to reach areas may include the concave surfaces of the airfoils, the root radii of the airfoils, and the platforms located on the support rail between each adjacent pair of airfoils. Moreover, current abrasive flow polishing methods may direct abrasive media directly at the certain regions of the airfoils (e.g., the leading edges) and this may lead to abrasive wear and structural damage in some cases.
Although one approach described in U.S. Patent Application number 2011/0047777 employs a mask to cover and protect specific regions of the airfoil cluster from contact with the abrasive media during abrasive flow machining, strategies for regulating the flow of abrasive media over the surfaces of the airfoil cluster to provide targeted surface polishing are still wanting. Clearly, a system is needed to control the direction and velocity of abrasive media flow over the surfaces of airfoil clusters during abrasive flow processes to ensure that targeted areas (i.e., the platforms and root radii of the airfoils) are polished to desired specifications.